1. Field of the Invention
Embodiments of the invention described herein pertain to the field of electric submersible pumps. More particularly, but not by way of limitation, one or more embodiments of the invention enable an apparatus, system and method for reducing gas intake in horizontal submersible pump assemblies.
2. Description of the Related Art
Fluid, such as oil or water, is often located in underground formations. In such situations, the fluid must be pumped to the surface so that it can be collected, separated, refined, distributed and/or sold. Conventionally, electric submersible pumps (ESPs) have been used to pump fluid from subsurface well bores. In an ESP assembly, well fluid enters the assembly through an intake section and is lifted to the surface by a multistage centrifugal pump. Centrifugal pumps impart energy to a fluid by conferring angular momentum to the fluid passing through the pump. The angular momentum converts kinetic energy into pressure, thereby raising the pressure on the fluid and lifting it to the surface.
FIG. 1 and FIG. 2 illustrate conventional intake sections of an ESP assembly of the prior art. FIG. 1 is a bolt-on intake of the prior art. FIG. 2 is an integral intake section of the prior art. As shown in FIGS. 1 and 2, intake sections are typically cylindrical in shape and allow for entrance of well fluid into the bottom of the centrifugal pump through intake ports. Slotted or perforated screens are sometimes placed around the outside of the intake to filter solids from the well fluid and/or to attempt to separate gas from the well fluid in reverse-flow designs. Intake sections are typically located between the pump and seal section in an ESP assembly. Rotatable shafts extend through the center of the ESP assembly, passing through the pump, intake section, seal section and electric motor. The shafts of the assembly components are connected in series; the motor turns the shafts, providing power to the pump.
In some instances, it is desirable to place an ESP horizontally in a well bore. Horizontal well bores including horizontally arranged ESP assemblies allow an increased amount of well fluid to be exposed to the pump assembly, which allows for increased fluid production as compared to vertical assemblies. Unfortunately, well fluid sometimes contains gas in addition to liquid. Conventional ESP assemblies are designed to handle fluid consisting mainly of liquid. When pumping gas-laden fluid, the gas and liquid may separate due to a lack of downhole pressure or low production inlet pressure. This is particularly true in horizontal wells, where well liquid may sometimes falls to the bottom part of the horizontal well, while gas builds up across the upper part of the well because of the difference in specific gravity between the gas and liquid.
A conventional horizontal ESP assembly is illustrated in FIG. 3. Conventional ESP assembly 100 includes ESP motor 130, ESP seal section 135, conventional ESP intake 140 and ESP centrifugal pump 145. Production tubing 150 carries produced well fluid to the surface. As shown in FIG. 3, conventional ESP assembly 100 is oriented horizontally in underground well 105, within casing 110. Perforations 115 allow well fluid to enter casing 110. During operation of ESP assembly 100, gas layer 120 forms on upper portion of assembly 100, and liquid layer 125 forms on the lower portion, due to the lack of downhole pressure and difference in specific gravity between gas and liquid in the well fluid. This separation of gas and liquid causes gas pockets that interfere with the fluid flow necessary for the pump to operate properly. If the amount of gas taken into the pump reaches typically around 10% to 15% by volume, the pump may experience a decrease in efficiency and decrease in capacity or head (slipping). If gas continues to accumulate on the suction side of the pump it may entirely block the passage of fluid through the pump. When this occurs the pump is said to be “gas locked” since proper operation of the pump is impeded by the accumulation of gas. Gas locking causes damage to the pump and a loss of production. In some instances even small amounts of gas in well fluid may cause an emulsion which is difficult for ESPs to handle. As a result, careful attention to gas management in submersible pump applications is needed in order to pump gas laden fluid from subsurface formations.
Conventionally gas separators have sometimes been used in an attempt to address the problems caused by gas-laden fluid in ESP applications. Gas separators attempt to remove gas from produced fluid prior to the fluid's entry into the pump. However it is often infeasible, costly or too time consuming to determine the correct type of pump and separator combination that might be effective for a particular well. Even if the correct arrangement is determined, the separator may not remove enough gas to prevent a loss in efficiency or gas locking. Alternatively, reverse-flow intakes have also been used to cause natural separation of gas prior to intake, but reverse-flow intakes are not effective in horizontal well applications. It would be an advantage in horizontal well applications if intake ports at the top of the intake section, where gas accumulates, could be closed to reduce gas intake, while the intake ports at the bottom, where the liquid accumulates, would remain open. In practice this concept has proven difficult to implement since during installation of a conventional ESP assembly, the assembly rotates about its longitudinal axis. As a result, the final radial orientation of the ESP assembly within the well is unknown prior to installation, and identifying which intake ports should be open and which intake ports should be closed has thus proven difficult.
In the case of an electric submersible pump (ESP), a failure of the pump or any support components in the pump assembly can be catastrophic as it means a costly delay in well production and having to remove the pump from the well for repairs. A submersible pump system capable of reducing gas intake would be an advantage in all types of submersible assemblies. Therefore, there is a need for an apparatus, system and method for reducing gas intake in horizontal electric submersible pump assemblies.